"I think the first breathalyzer on the market will be a simple 'yes' or 'no' for the presence of THC at the time of the test, and in that sense it won't provide a quantitative evidential measure," Barry Knott, the chief executive of Lifeloc, which already makes alcohol breathalyzers, told Reuters. That's cool and all, except existing DWI for cannabis tests -- both urine and blood analysis -- already do this with greater accuracy. There's also the issue with THC's fat-soluble nature that allows it to remain in the user's bloodstream for up to a month after ingestion. Having a device that only says "yes" or "no" doesn't actually add any value to the police officer's investigation when the machine's determination could be based on biomarkers more than a fortnight old. Granted, the Cannabix device is reportedly designed to detect THC up to two hours after it's been smoked but there's no word on how (or whether) it will work for ingested or vaporized cannabis. In fact, the company has been rather tight lipped in general about how the device operates, save for pointing out that it is patent pending.
Then there's the whole mess of differing levels of impairment between states. In Washington and Montana 5 nanograms/milliliter (ng/mL) of THC counts as "too stoned to drive." In Pennsylvania, that limit is 1 ng/mL. And as Nicholas Lovrich, a political scientist at Washington State University, told Reuters, these limits are based more on politics than on science. So until both the science and policy surrounding cannabis advance beyond their present states, don't expect these devices to be entered into evidence in your DUI case anytime soon (unlike that Mass Roots post of you hotboxing the car 20 minutes before getting pulled over).
This article by Andrew Tarantola originally ran on Engadget, the definitive guide to this connected life.